1. Field of the Invention
The present invention concerns a process for analyzing a light sensitive or photosensitive device having an interline-transfer structure. It also concerns an apparatus for carrying out said process.
2. Description of the Prior Art
Photosensitive devices having an interline-transfer structure are well known to those skilled in the art. In particular, they are described in the publication of CH. SEQUIN and N. F. TOMPSETT entitled "Charge Transfer Devices", pages 156 to 166.
As shown in FIGS. 1 and 2(a) appended hereto, a photosensitive device having an interline-transfer structure is essentially constituted by a photosensitive matrix 1 comprising N column of M photosensitive points P, analog shift registers 2 provided between any two adjacent columns of M photosensitive points as well as a reading register 3.
In a more detailed manner, the photosensitive points P of a column are connected to M stages of an analog register 2, preferably constituted by a charge transfer register of the CCD type, having parallel inputs and series output, through the intermediary of a potential barrier various means may be utilized to realize this potential barrier. Thus, as represented on FIG. 2(a), the CCD register 2 is of the volume transfer type, i.e. the transfer channel of the register 2 has received an implantation of impurities of the type opposite that of the substrate in order to realize a bulk channel TV, for example, an N type impurities implantation for a P type silicon substrate. The transfer channel TV is separated from the photosensitive points P constituted by photodiodes in the embodiment represented, by a surface transfer zone 4, namely a zone free from doping, the channel and the said surface transfer zone 4 both being convered with storage electrodes ES of the CCD register. For this reason a potential barrier, the level of which varies in function of the voltage applied to electrodes ES, is obtained between photodiodes P and register 2. The potential barrier can also be realized through the use of a supplementary electrode provided between the photodiodes P and register 2 which, in this case, can be of the surface transfer type or of the volume transfer type.
The output of the various registers 2 is sent, through the intermediary of passage gate G.sub.p into a reading register 3 positioned perpendicular to registers 2. Register 3 is generally constituted by an analog shift register that supplies data received in parallel in series mode. This register is preferably a CCD type transfer register.
Furthermore, the different columns are separated from one another by an insulating barrier 5 realized by an oxide overthickness in the embodiment represented in FIG. 2(a).
The different image integration operations at the level of the photosensitive matrix 1, then of the reading of the charges corresponding to this image in order to obtain at the output of register 3 the video analysis signal of the optical image received on the photosensitive zone, are analyzed as follows:
During line scanning time, the duration of which is generally 20 milliseconds, signal charges Q.sub.S are integrated in photosensitive points P. Thereafter, signal charges Q.sub.S stored in the different photosensitive points are transferred to corresponding CCD registers. To this end a high potential .0..sub.CCD is applied to electrodes ES of register 2, so as to obtain the potential curve referenced A in FIG. 2(b), which eliminates the potential barrier between the photosensitive points P and the corresponding registers 2. Once the transfer is complete, control phases .0..sub.CCD constituted by square-wave voltage, are applied to the registers 2 providing successively potential curves referenced B and C. The potential barrier is thus recreated between the photosensitive points and registers 2, which allows a fresh integration of signal charges in the photodiodes. During this time, registers 2 effect the shift of the charges contained in ach stage towards read-register 3 that transfers the said charges towards a reading-stage between any two successive transfers issuing from the register 2.
However, with this mode of operation, an important remanence phenomenon is observed, due to the fact that the transfer of the signal charges from the photodiodes towards registers 2 terminates in low inversion conditions as will be explained in detail with reference to FIG. 3.
FIG. 3 is a curve illustrating the transfer of the signal charge Q.sub.S stored in a photodiode P towards the corresponding CCD register. It demonstrates the evolution of this charge as a function of time. This curve comprises a substantially vertical portion corresponding to a transfer in high inversion conditions, then a bend and finally a substanially horizontal portion corresponding to a transfer in low inversion conditions.
It is presumed that a quantity of charges Q.sub.S has been integrated in a photodiode P. The characteristic Q(t) of FIG. 3 is thus repeated at point .circle.1 , due to the residual charge Q.sub.r remaining in the photodiode. Since the capacity of the photodiode is very small, point .circle.1 is positioned in the high inversion zone. After reading-time TL the charge Q.sub.S was transferred to register .circle.2 . The operating point of the photodiode is present at point .circle.2 of the curve, i.e. in the bend of the curve, and the photodiode only contains the residual charge Q.sub.r. In this case, the transfer of the signal charge Q.sub.S has been effective, since it is carried out in the high inversion range. In the following scanning line, if the luminous object was not displaced, a fresh charge Q.sub.S has been integrated in the photodiode. It is again transferred and the charge of the photodiode thus evolves between two equilibrium points .circle.1 and .circle.2 .
However, if the luminous object is displaced and if the photodiode involved does not receive any more luminous data, at the reading of the following frame realized in time TL, the charge of the photodiode evolves from point .circle.2 towards point .circle.3 . This consequently results in the reading of interference data Q'.sub.S. If the absence of lighting continues, an evolution from point .circle.3 towards point .circle.4 is observed with the reading of an interference charge Q".sub.S and so on until the low inversion current is set off by the obscurity current of the photodiode. The reading of the charges Q'.sub.S, Q".sub.S . . . thus constitutes the remanence phenomenon. This phenomenon is particularly prejudicial at low light level, since in this case the charge Q'.sub.S, Q".sub.S . . . have an amplitude as high as the signal.
The present invention is thus aimed at overcoming this drawback by proposing an analysis process of an interlaced scanning type photosensitive device that allows to reduce considerably the remanence phenomenon.